The media stumbles over a basic aerodynamic aspect of helicopter flight.
I was on Twitter Thursday evening when manp, one of my Twitter friends, tweeted:
So, what is this ‘vortex’ condition with ‘higher than expected temperatures’??? @mlanger any idea?
To be honest, I had no clue what he was talking about. But I Googled “vortex condition with higher than expected temperatures” (don’t you love Google?) and saw an article about the helicopter that went down during the Bin Laden assault in Pakistan. Moments later, manp sent me a link to a Bloomberg article titled “Helicopter Carrying SEALs Downed by Vortex, Not Mechanical Flaw or Gunfire.” The first paragraph read as follows:
A United Technologies Corp. (UTX) Black Hawk helicopter carrying U.S. Navy SEALs to Osama Bin Laden’s hideout was downed by an air vortex caused by unexpectedly warm air and the effect of a high wall surrounding the compound, not mechanical failure or gunfire, according to U.S. officials and a lawmaker.
Whoa. What a mishmash of information. You have to read further into the article where the phenomena they’re trying to explain — vortex ring state — is explained at least two more times by people who actually have a clue what it is. But that first paragraph sure is misleading. It makes it seem as if there was come kind of weird warm air vortex in the compound that brought the helicopter down.
Any vortexes, however, were caused by the helicopter itself. My educated guess of what happened, based on this article and knowledge of helicopter aerodynamics, is this:
As the helicopter was descending inside the 18-foot walls — a descent that was likely nearly vertical — it encountered a setting with power — or vortex ring state — condition. This occurs when the helicopter settles into its own downwash. This may have been made worse by the change in the flow of air due to those 18-foot walls — as suggested in the article. It may also have been made worse by the outside air temperature being warm.
This image from the FAA’s Rotorcraft Flying Handbook helps illustrated what the vortexes are and how they manifest themselves in a hover far above the ground and close to the ground:
As the Rotorcraft Flying Handbook explains:
Vortex ring state describes an aerodynamic condition where a helicopter may be in a vertical descent with up to maximum power applied, and little or no cyclic authority. The term “settling with power” comes from the fact that helicopter keeps settling even though full engine power is applied.
In a normal out-of-ground-effect hover, the helicopter is able to remain stationary by propelling a large mass of air down through the main rotor. Some of the air is recirculated near the tips of the blades, curling up from the bottom of the rotor system and rejoining the air entering the rotor from the top. This phenomenon is common to all airfoils and is known as tip vortices. Tip vortices consume engine power but produce no useful lift. As long as the tip vortices are small, their only effect is a small loss in rotor efficiency. However, when the helicopter begins to descend vertically, it settles into its own downwash, which greatly enlarges the tip vortices. In this vortex ring state, most of the power developed by the engine is wasted in accelerating the air in a doughnut pattern around the rotor.
In addition, the helicopter may descend at a rate that exceeds the normal downward induced-flow rate of the inner blade sections. As a result, the airflow of the inner blade sections is upward relative to the disc. This produces a secondary vortex ring in addition to the normal tip-vortices. The secondary vortex ring is generated about the point on the blade where the airflow changes from up to down. The result is an unsteady turbulent flow over a large area of the disc. Rotor efficiency is lost even though power is still being supplied from the engine.
There are three ways to recover from settling with power once you’re in it:
- Cut power – you can’t settle with power if you don’t have power. This is usually not a good option when you’re very close to the ground.
- Lower the collective – this reduces the blade pitch. This is also not a good idea close to the ground, since it will result in a descent.
- Get some lateral airspeed – this breaks you out of the vortex ring state so you’re not settling in your own downwash. This is not possible when you’re surrounded by an 18-foot wall.
(They train us to recover from settling with power using a combination of the second two methods, but we always practice at altitude, since you can get a good descent rate going if you’re really into it. Indeed, settling with power is a serious danger during aerial photo missions requiring hovering at high density altitudes or heavy weights.)
So the pilot did the only thing he could: land hard. Fortunately, although his hard landing damaged the helicopter, it didn’t cause injuries to to men on board. They were able to complete their mission and come home safely. And they left a souvenir lawn ornament in Bin Laden’s yard.
I realize that this is a pretty complex topic and it’s probably not reasonable to expect the press to get it right. But I personally believe that all technical content published in the media should be reviewed by an expert — or at least someone knowledgeable — to make sure it’s not misleading or unclear to the layperson who will read it.
manp is a pilot — although not a helicopter pilot — and he couldn’t figure out what they were talking about. I can only imagine how much that opening paragraph confused the average reader.